The present invention relates to a program control system especially adapted for the operations such as automatic welding, automatic coating and so on which require continuous controls.
Various industrial robots have been recently developed and used in various fields. In the so-called "teaching mode", the manipulator assembly of an industrial robot is taught to trace a prescribed path to perform a series of operation steps in succession. Specified position information, that is the coordinates of the specified points of the prescribed path at which the specified steps of operations must be performed are stored in a storage device. In the so-called "repeat cycles", the industrial robot performs a series of operational steps taught in succession.
To control the industrial robots of the type described, the so-called point-to-point control system has been widely employed. In the repeat cycle, the command signal for causing the industrial robot to perform the first step is read out from the storage device, and is compared with the actual or present position information representative of the actual or present position of the manipulator assembly of the industrial robot, so that manipulator may be actuated or displaced in the direction in which the difference between the command signal and the actual or present position signal may become zero. When the difference becomes zero, that is when the manipulator reaches the point designated by the command signal, the manipulator is stopped and then the next command signal is read out and compared with the actual or present position signal so that the manipulator is actuated or displaced in the direction in which the difference between the two signals may become zero. In like manner, the above operations are cycled as the manipulator is displaced from one specified point to another.
As is clear from the above explanation of the point-to-point control system, it suffices to control the manipulator assembly of an industrial robot so that the manipulator assembly may be displaced from one specified point to another of a prescribed path in space, and where the specific path the manipulator assembly follows as it moves from one specified point to the next is immaterial. The industrial robots employing the point-to-point control system are best adapted for use in the spot welding operations, assembly work and so on, in which the path along which the manipulator assembly traces from one specified point to the next is immaterial so that considerable man-power saving and the improvement in operation efficiency may be expected.
However, there arise various problems when the industrial robots employing the point-to-point control system are employed in the operations such as arc weldings, coating and so on in which the manipulator assembly of the robot must correctly trace a prescribed path in space continuously. Therefore there has been devised and demonstrated a program control system for industrial robots or manipulators in which a prescribed path to be traced by the robots or manipulators is considered as consisting of the totality of points of a prescribed path, so that the manipulator is caused to move from one specified point to the next to trace the whole prescribed path. But this system has a distinct disadvantage in that a tremendous number of sets of the coordinates of the specified points of a prescribed path must be stored in a storage device, thus resulting in intolerable complexity in the so-called "teaching mode". Furthermore, a large volume of the position signals representing the coordinates of the specified points must of course be read out in the repeat mode, so that it becomes extremely difficult to attain highspeed control.
One of the objects of the present invention is to provide an improved program control system especially adapted for use in the operations in which the manipulator assembly of an industrial robot must correctly and continuously trace a prescribed path in the space as in the case of arc weldings, coatings and so on.
Another object of the present invention is to provide an improved program control system of the type described without causing complexity in the teaching mode and without increasing of the capacity of a storage device.
Briefly stated, according to the control system employing a program control system in accordance with the present invention, a manipulator assembly is caused to displace itself along a line segment of a unit length of a prescribed path from one specified point to the next. The position information or the coordinates of each specified point of the prescribed path is converted into digital signals by an encoder, which is provided for each axis or degree of freedom of the manipulator assemby, and the digital signals are stored in a storage device. In the repeat mode one set of the specified position information or coordinates of the adjacent specified points are read out substantially simultaneously at a predetermined time interval, and are applied to a linear interpolation unit so that the position command signals or coordinates of the interpolation points between the adjacent specified points may be sequentially obtained. The position command signals are sequentially compared, at a predetermined time interval, with the actual or present position signal representing the actual or present position of the manipulator assembly so that the manipulator may be displaced at a velocity corresponding to the difference between the position command signal and the actual or present position signal in the direction which the difference may become zero or may be eliminated. Thus the manipulator assembly may correctly trace a prescribed path.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of one preferred embodiment thereof taken in conjunction with the accompanying drawing.